1. Field of the Invention
This invention relates generally to temporary cardiac assist devices used to assist the operation of a failing, traumatized or infarcted heart for a limited period until either the heart recovers or more definitive treatment can be provided. In particular, it relates to so-called intra-aortic balloon pumps. Such a pump does not require major thoracic surgery to connect it to the circulation but is a collapsible structure which may be introduced into an easily accessible large artery, such as a femoral, and may then be guided into some portion of the aorta, usually the thoracic aorta, where it can be employed to assist the left side of the heart, the side most frequently in need of assistance and the driving force behind the systemic circulation. In the usual "counterpulsation" mode of employment, the balloon is pneumatically inflated during diastole to increase blood pressure and deflated during systole to lower the pressure load upon the ventricle. This device and its mode of operation was described in a paper by Moulopolous, Topaz and Kolff, "Diastolic Balloon Pumping in the Aorta--A Mechanical Assistance to the Failing Circulation", American Heart Journal (1962) 63, p. 669.
2. Prior Art Problem
Since intra-aortic balloon pumps can be applied with relatively minor surgery and fairly standard vascular catheterization procedures, and afford some useful assistance to the left heart, they are well regarded. However, they provide much less pumping assistance than one would desire, for at least three reasons: first, the conveniently available volume within the aorta is not large compared to the desired stroke volume of the heart; second, the necessity to avoid occlusion of important arteries such as the carotids and renals, and erosion of atherosclerotic plaques, further limits the size of a pump balloon; and third, the elastic compliance of the aortic walls makes the effective pumping displacement of the balloon less than its geometric displacement. Many inventions have been addressed to the alleviation of this problem: For example, U.S. Pat. No. 3,504,662 to R. T. Jones shows how to attain better pump fluid dynamics, and U.S. Pat. No. 3,692,018 to Goetz and Goetz shows how to direct the limited available flow predominantly to the critical brain and heart circulations. Still, there is no complete solution to the problem of adequate pump output. Also, since the counterpulsation mode requires synchronization with the natural heart, there is a further problem when the available ECG signal is weak, uneven or missing, as may occur in ill patients.